1. Field of the Invention
This invention relates to a novel polyacetal resin blend useful in the injection molding of hollow tubular structures such as cigarette lighter bodies. The present invention is also concerned with a process for forming dimensionally stable hollow tubular bodies by injection molding a novel oxymethylene polymeric blend comprising oxymethylene polymers of differing melt index.
2. Description of the Prior Art
Oxymethylene polymers having recurring --CH.sub.2 O-- units, have been known for many years. These polymers may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde.
Oxymethylene polymers of exceptional toughness and high molecular weight may be prepared by polymerizing trioxane in the presence of a cationic catalyst such as boron fluoride-containing catalysts, including boron fluoride, itself and boron fluoride coordination complexes with organic compounds in which oxygen or sulfur is the donor atom.
The usefulness of oxymethylene polymers in molding compositions is well known. Oxymethylene polymers are distinguished by a number of excellent properties so they are suitable for a variety of industrial applications. Many of the desirable properties of oxymethylene polymers result from the fact that these polymers are highly crystalline. However, microscopic examination of thin sections of such polymers also shows that there is a non-homogeneous coarse spherulitic structure which when forming during solidification of a melt, produces different shrinkage values, distortions, and internal stresses, which lead to variations in the dimensions of molded articles manufactured therefrom. Accordingly, it is well known to incorporate nucleating agents into oxymethylene polymer compositions to control and render the crystallization of the oxymethylene polymer more uniform. Many nucleating agents are known in the art. U.S. Pat. No. 3,775,363 which is just illustrative of such art discloses acicular calcium metasilicate as a nucleating agent for oxymethylene molding resin.
One commercial application of oxymethylene polymers is in the mass production of cigarette lighter bodies. Cigarette lighter bodies are hollow cylinders of circular or elliptical cross-section which contain a reinforcing rib positioned along the diameter of the base. The lighter bodies serve as the supply for the lighter fluid and as the support for the flame-forming mechanism. The cigarette lighter bodies are manufactured by injection molding the rib-reinforced lighter bodies from the oxymethylene polymer in one piece. To mass produce such articles, the lighter bodies are ejected from the mold prior to being completely hardened. The lighter bodies, thus, are cooled and completely hardened outside the mold. Upon cooling, a separate base plug is inserted into the end of the lighter body and sonic welded thereto. It can be readily seen that in mass producing such cigarette lighter bodies from oxymethylene polymers by injection molding, it is necessary that the oxymethylene polymer chosen continuously yield lighter bodies of uniform base dimensions to insure that the base plug can be snugly fit into the end of the lighter body on a mass production scale. Avoiding variations in the dimensions of the molded ligher bodies is critical if mass production is to be maintained with a minimum of waste.
Although dimensionally stable cigarette ligher bodies have been mass produced by injection molding oxymethylene homopolymer, it has been found more difficult to produce cigarette lighter bodies with uniform dimensions when injection molding oxymethylene copolymers on a mass scale. Oxymethylene copolymers as defined herein comprise a major proportion of oxymethylene units and a minor proportion of oxyalkylene units, in which the alkylene contains at least two carbon atoms, such as ethylene oxide. Thus, it has been found that when injection molding oxymethylene copolymers into cigarette lighter bodies, the dimensions of the tubular body often become distorted during the cooling and final hardening stage after the lighter bodies have been ejected from the mold. The distortion results from melt flow along the diametrically placed reinforcing rib prior to completion of hardening and crystallization of the polymeric lighter body. It has been unexpectedly found that relatively high molecular weight oxymethylene copolymers do not crystallize at a sufficient rate to avoid melt flow along the rib and consequent distortion of tubular diameter.
Accordingly, another physical property of oxymethylene polymers which must be considered in the manufacture of molded articles, including cigarette lighter bodies, is the ease of flow of such polymers. Thus, for some applications, it is desired to have polymers which flow readily at elevated temperatures while for other applications it is desired to have polymers which resist such flow. The melt index, which is related to molecular weight, provides an indication of the ease of flow of a polymer.
The melt index is determined by heating a sample of a polymer in a standard cylinder to a standard temperature of 190.degree. C. and forcing it under a standard load of 2.160 kg. through a standard orifice of 0.0825 inch diameter and 0.315 inch long for a standard period and weighing the polymer passing through the orifice during this period. The results are recorded in grams per 10 minutes. The test is described in detail in ASTM D-1238-57T.
The melt index (10X) is generally used when melt index values are low and is determined in an identical manner except that the standard load is increased 10 fold to 21.60 kg.
It can readily be seen that in forming molded articles from oxymethylene polymers, a particular combination of physical properties of the oxymethylene polymer must be provided to meet the desired design specifications of the articles, whether to meet particular mechanical properties such as impact strength and the like and/or to provide dimensionally stable articles where avoiding variations in the dimensions of molded units is critical.
It is thus a primary object of the present invention to provide an oxymethylene copolymer molding composition having desired physical properties so as to be injection molded into cigarette lighter bodies which can meet strength requirements and critical dimension requirements.
It is another object of the invention to provide an oxymethylene copolymer molding composition which can be injection molded into cigarette lighter bodies of uniform dimension so that such lighter bodies can be mass produced.